Abstract
Large sediment fluxes from mountain belts have the potential to cause megafans to prograde into the neighbouring sedimentary basins. These mechanisms have been documented based from numerical modelling and stratigraphic records. However, little attention has been focused on inferring temporal changes in the concentrations of supplied sediment from coarse-grained deposits. Here, we extract changes of this variable in the field from a Late Oligocene, c. 4 km-thick suite of fluvial conglomerates situated in the North Alpine foreland basin, which evolved in response to the tectonic and erosional history of the Alps. We measure a decrease in channel depths from >2 m to <1 m and an increase in the largest grain sizes from <15 cm to >20 cm from the base to the top of the suite. These constraints are used to calculate an increase in fan surface slopes from <0.3° to >1.0° based on the Shields criteria for sediment entrainment. We combine slope and bulk grain size data with the Bagnold equation for sediment transport to infer higher concentrations of the supplied sediment. We use these shifts to propose a change towards faster erosion and a steeper landscape in the Alpine hinterland, driven by mantle-scale processes beneath the Alps.
Highlights
The coarsening- and thickening-upward megasequence suggests that the depositional style changed through time from a distal position to a more proximal environment on the megafan[3,4,21,29]
Supporting evidence is provided by the change in the stratal architecture, the sedimentary fabric and the trend toward less rounded clasts from the base to the top of the Rigi conglomerates
Shifts towards a more proximal environment, where conglomerate beds are more frequent in the stratigraphic suite, are observed along the steep ridge to the NW of Rigi-Kulm, exposing sediments of parts α through γ in their vertical position
Summary
The coarsening- and thickening-upward megasequence suggests that the depositional style changed through time from a distal position to a more proximal environment on the megafan[3,4,21,29]. Supporting evidence is provided by the change in the stratal architecture, the sedimentary fabric and the trend toward less rounded clasts from the base to the top of the Rigi conglomerates This change in sedimentary fabric and depositional processes was associated with (i) an upsection trend towards a steeper slope of the fan surface, (ii) an increase of the largest clasts, and (iii) a decrease in channel depths. Such a change in the stratigraphic record could either be explained by the exposure situation where c. While we cannot fully exclude the possibility that the exposure condition partially explains the aforementioned stratigraphic architecture, the coarsening- and thickening-upward record does include a progradational trend of the Rigi megafa
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